We have migrated Mailenable from old server to new server, and also we have configured all the configuration same as old server. We are using MailEnable Standard Edition (V9) and Installed on Windows 2012. When we try to send mail from our new MailEnable server it is working fine with Yahoo.com, Hotmail.com, Outlook.com, rediffmail.com and other external domains too.. But for some few domains it is not working exampe : gmail.com, msrit.edu etc..

The log snippets you provided in your last post are not associated with problems sending to Gmail for the 421 error in your original post. They are log snippets relating to the relay being denied when sending from MailEnable. You need to search for the SMTP activity log file F887416D8A7041D5841426209ADA8343.MAI where the 421 error was returned from the Gmail. Have you reviewed the article I provided earlier and ensured that you have all the required prerequisites set when sending to Gmail servers? I.e: PTr record, SPF record, DKIM etc.